sedimentary
Chert (Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Smoky grey/tan, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6
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Physical properties
Hardness: 7 (Mohs scale), Color: Smoky grey/tan, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6
Formation & geological history
Formed from the precipitation of silica in sedimentary environments, often replacing organic material or originating from the shells of silica-secreting organisms like diatoms or radiolarians.
Uses & applications
History was primarily used for stone tools (knapping) due to its conchoidal fracture. Modernly used for road surfacing, fire-starting kits, and as a component in jewelry/lapidary.
Geological facts
This specimen shows clear conchoidal fracturing and striations common in chert. Ancient humans relied heavily on this rock for making arrowheads, scrapers, and knives.
Field identification & locations
Identify in the field by its sharp edges, waxy feel, and ability to scratch glass. It is often found as nodules within limestone or chalk beds.
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Brecciated Chert (Microcrystalline Silica)
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